The present invention relates to a method of producing a well-integrated multilayer material comprising at least two adjacent layers of mutually sparingly compatible materials, in which method the two sparingly compatible material layers are permanently united to one another by means of an adhesive or binder which is applied between the layers.
Within packaging technology, use is often made of packages of the single-use type for transporting liquid foods, and a very large group of these so-called single-use disposable packages is produced from a packaging material comprising a plurality of individual material layers which are bonded to one another with good bonding strength for the formation of a well-integrated multilayer material (a packaging laminate).
The point of departure of the composition of the packaging laminate is to impart to the package the best possible protection for the product that is to be packed, at the same time as the package itself must be mechanically strong and stable, so as to make for rational handling and consumer-friendly use. The predominant group of such configurationally stable, easy-to-use disposable packages is produced from a packaging laminate comprising a configurationally rigid core layer of paper or paperboard and outer, liquidtight coatings of plastic, preferably polyethylene.
However, a packaging laminate consisting solely of paper or paperboard and polyethylene possesses no tightness properties vis-à-vis gases, in particular oxygen gas, and must therefore be supplemented with at least one additional layer (a so-called gas barrier) so as to make possible the production of gas-tight packages.
One well-known example of such a gas barrier is a thin (approximately 5-10 xcexcm) foil of aluminium (so-called Alifoil) which is applied between the paper or paperboard layer of the packaging laminate and the one outer polyethylene coating. Other prior art examples of gas barriers are certain polymer materials, such as polycarbonate, polyamide, polyester and copolymers of ethylene and vinyl alcohol (EVOH) which, like the Alifoil, are applied between the paper or paperboard layer and the one polyethylene coating. Thin (approximately 200-500 xc3x85) vacuum-deposited layers of metal (aluminium) and other inorganic materials (silicon oxide) have also been proposed in the art as gas barriers in packaging laminates of the above-described type. Among these examples, the Alifoil still retains its paramount position as a gas barrier in packages for oxygen gas sensitive products such as, for example, juice, wine, cooking oil etc., even though the alternative gas barrier materialsxe2x80x94in particular EVOHxe2x80x94are being used to an increasing extent.
Packaging laminates or corresponding multilayer materials are generally produced by lamination and/or (co-)extrusion in one or more process stages through which individual material layers are united to one another to form a well-integrated packaging material. The intimate integrity of the packaging material, which is a vital precondition for the best possible product protection, naturally presupposes that the individual material layers are mutually compatible and unifiable with one another and/or may be united to one another with such bonding strength that they do not delaminate. When the packaging laminate comprises two adjacent layers of mutually sparingly compatible or incompatible materials, for example olefin plastic/ polyester or olefin plastic/EVOH, as may be the case in packaging laminates for oxygen gas-tight packages, the sought-for integrity will be attained using so-called adhesion plastics or similar binders (adhesives) which are applied between the two material layers. The requirement placed on such adhesion plastics and binders is, on the one hand, that they be able, with good bonding strength, to bond to both of the surrounding material layers and, on the other hand, that they themselves form a well-integrated material layer possessing good internal cohesive properties.
Examples of binders of the adhesive type are lacquers, glues and primers, while one known example of a binder of the adhesion plastic type is SURLYN(copyright) which is an ionomeric product of ethylene methacrylic acid (EMMA) and zinc acetate. Binders of the adhesive type arc often associated with organic solvents which, not least for environmental reasons, it is desirable to avoid, while binders of the adhesion type are, granted, solvent-free, but often associated with high costs.
One object of the present invention is therefore to propose a method of producing a well-integrated multilayer material of the type described by way of introduction without using either solvent-based or expensive binders or adhesives.
A further object of the present invention is to realise a simple and efficient method of producing such a well-integrated multilayer material on a rational industrial scale.
Still a further object of the present invention is to produce a well-integrated packaging laminate for packages possessing superior oxygen gas barrier properties.
Solution
These and other objects and advantages will be attained according to the present invention by a method of the type disclosed in the characterizing clause of appended Claim 1. Expedient embodiments of the method according to the present invention have further been given the characterizing features as set forth in the appended subclaims.
According to the invention, it has surprisingly been found that the favourable properties of prior art binders of the adhesive typexe2x80x94such as low material costsxe2x80x94may advantageously be combined with the favourable properties of known adhesion plastic type binding agents, in the form of the avoidance of organic solvents, at the same time as the unfavourable properties of the prior art adhesives and binders may effectively be reduced or wholly obviated.
In one practical embodiment of the present invention, the high viscosity binder component consists of a homopolymer of polyethylene, while the low viscosity binder component preferably consists of a single SURLYN(copyright) (a neutralized, metal modified ethylene-carboxylic acid copolymer resin), or a polyethylene based adhesive. The mixing ratio between polyethylene and SURLYN(copyright) amounts to between 5 and 20 percent of the total weight of the mixture. A binder consisting of approximately 5-20 percent SURLYN(copyright) and approximately 95-80 percent polyethylene may be effective as a binder for permanently uniting a material layer of polyester (barrier layer) and a material layer of polyethylene for the formation of a well-integrated packaging laminate by the method according to the invention.
In a further practical embodiment of the present inventions for permanently uniting. a material layer of EVOH (ethylene vinyl alcohol copolymer) and a material layer of polyethylene, the high viscosity binder component consists of polyethylene, while the low viscosity component consists of a polyethylene based adhesion plastic with suitable functional groups which bond to corresponding functional groups in EVOH. A person skilled in the art will be well aware of the appropriate adhesion plastics, for example anhydride modified polyethylene, and may readily select from among them.
According to the invention, the mixing ratio between the high viscosity binder component (polyethylene) an the low viscosity binder component (adhesion plastic with suitable functional groups) may vary, but is preferably selected such that the quantity of the low viscosity binder component amounts to approximately 5-40 percent of the total weight of the binder. A binder consisting essentially of approximately 5-40 percent low viscosity component (adhesion plastic with suitable functional groups) and approximately 95-60 weight percent high viscosity binder component (polyethylene) may function well as a binder for permanently uniting a material layer of EVOH (barrier layer) and a material layer of polyethylene, for the formation of a well-integrated packaging laminate by the method according to the present invention.